Optical transmitter, optical communication system, and optical communication method

ABSTRACT

If a configuration is employed in which modulation schemes used for an optical communication system can be switched depending on transmission conditions, the power consumption increases and the control becomes complex; therefore, an optical transmitter according to an exemplary aspect of the present invention includes an encoding means for encoding digital signals to be transmitted under a predetermined transmission condition over an optical carrier wave by using one of a plurality of encoding methods; an encoding control means for selecting a predetermined encoding method corresponding to the predetermined transmission condition from among the plurality of encoding methods and causing the encoding means to operate in accordance with the predetermined encoding method; a mapping means for mapping output bit signals output from the encoding means to modulation symbols; and an optical modulation means for modulating the optical carrier wave based on symbol signals output from the mapping means.

TECHNICAL FIELD

The present invention relates to optical transmitters, opticalcommunication systems, and optical communication methods, in particular,to an optical transmitter, an optical communication system, and anoptical communication method that perform optical coded modulation usingdigital signals.

BACKGROUND ART

It is important in an optical communication system using optical fibersto increase the receiving sensitivity and the frequency utilizationefficiency per optical fiber in order to achieve a long-distance andlarge-capacity communication. It has been proposed to switch variousmodulation schemes that differ in the reachable transmission distanceand the frequency utilization efficiency depending on a transmissiondistance and a transmission capacity that are required because theoptical communication has a trade-off relationship between increase intransmission distance and improvement in frequency utilizationefficiency. Examples of modulation schemes that differ in the reachabletransmission distance and the frequency utilization efficiency includeBPSK (binary phase shift keying), QPSK (quadrature phase shift keying),8QAM (quadrature amplitude modulation), and 16QAM.

One example of optical transmitters that are used switching modulationschemes is described in Patent Literature 1. The related opticaltransmitter described in Patent Literature 1 includes a clientaccommodation unit, a variable frame mapping unit, a variable coder, anoptical modulation unit, and a transmission method setting unit.

The client accommodation unit terminates a client signal transmittedfrom a client. The variable frame mapping unit allocates the clientsignal terminated by the client accommodation unit to a predeterminedtransmission frame. At this time, the variable frame mapping unitperforms mapping in accordance with a transmission method selected bythe transmission method setting unit.

The variable coder generates a modulation signal used for carrying thetransmission frame generated by the variable frame mapping unit. At thistime, the variable coder generates the modulation signal in accordancewith the transmission method selected by the transmission method settingunit. The optical modulation unit generates a modulated optical signalfrom the modulation signal generated by the variable coder and outputsthe modulated optical signal. At this time, the optical modulation unitgenerates the modulated optical signal in accordance with thetransmission method selected by the transmission method setting unit.

The transmission method setting unit selects a transmission methodcorresponding to a transmission rate of the client signal from among aplurality of transmission methods provided by the optical transmitter.In addition, the transmission method setting unit notifies the variableframe mapping unit, the variable coder, and the optical modulation unitof transmission method information indicating the selected transmissionmethod.

As mentioned above, the related optical transmitter is configured totransmit a client signal using the transmission method corresponding tothe transmission rate of the client signal. It is said that thisconfiguration enables the transmission efficiency to be improved becausethe transmission amount of a useless signal is small even though thetransmission rate of the client signal is low.

Other related techniques are described in Patent Literature 2 to PatentLiterature 4.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent Application Laid-Open Publication No.    2011-250291 (Paragraphs [0016] to [0025])-   PTL 2: Japanese Patent Application Laid-Open Publication No.

2003-087345

-   PTL 3: Japanese Patent Application Laid-Open Publication No.    2009-105748-   PTL 4: Japanese Patent Application Laid-Open Publication No.    2011-514736

Non Patent Literature

-   NPL 1: Leonardo D. Coelho and Norbert Hanik, “Global Optimization of    Fiber-Optic Communication Systems using Four-Dimensional Modulation    Formats”, in European Conference on Optical Communications (ECOC    2011), Technical Digest, paper Mo.2.B.4.-   NPL 2: J. Renaudier, A. Voicila, O. Bertran-Pardo, O. Rival, M.    Karlsson, G. Charlet, and S. Bigo, “Comparison of Set-Partitioned    Two-Polarization 16QAM Formats with PDM-QPSK and PDM-8QAM for    Optical Transmission Systems with Error-Correction Coding”, in    European Conference on Optical Communications (ECOC 2012), Technical    Digest, paper We.1.C.5.

SUMMARY OF INVENTION Technical Problem

If a single optical transmitter is used switching between a plurality ofmodulation schemes such as BPSK, QPSK, 8QAM, and 16QAM as theabove-mentioned related optical transmitter, it is necessary toimplement a signal processing circuit with a plurality of algorithms andbit precisions that correspond to the plurality of modulation schemes.Consequently, there has been the problem that the power consumption ofthe optical transmitter and an optical receiver increases and thecontrol of them becomes complex.

As mentioned above, there has been the problem that, if a configurationis employed in which modulation schemes used for an opticalcommunication system can be switched depending on transmissionconditions, the power consumption increases and the control becomescomplex.

The object of the present invention is to provide an opticaltransmitter, an optical communication system, and an opticalcommunication method that solve the above-mentioned problem that, if aconfiguration is employed in which modulation schemes used for anoptical communication system can be switched depending on transmissionconditions, the power consumption increases and the control becomescomplex.

Solution to Problem

An optical transmitter according to an exemplary aspect of the presentinvention includes an encoding means for encoding digital signals to betransmitted under a predetermined transmission condition over an opticalcarrier wave by using one of a plurality of encoding methods; anencoding control means for selecting a predetermined encoding methodcorresponding to the predetermined transmission condition from among theplurality of encoding methods and causing the encoding means to operatein accordance with the predetermined encoding method; a mapping meansfor mapping output bit signals output from the encoding means tomodulation symbols; and an optical modulation means for modulating theoptical carrier wave based on symbol signals output from the mappingmeans.

An optical communication system according to an exemplary aspect of thepresent invention includes an optical transmitter configured to send outa modulated optical signal to an optical transmission medium; and anoptical receiver configured to receive the modulated optical signalpropagated through the optical transmission medium, wherein the opticaltransmitter includes an encoding means for encoding digital signals tobe transmitted under a predetermined transmission condition over anoptical carrier wave by using one of a plurality of encoding methods, anencoding control means for selecting a predetermined encoding methodcorresponding to the predetermined transmission condition from among theplurality of encoding methods and causing the encoding means to operatein accordance with the predetermined encoding method, a mapping meansfor mapping output bit signals output from the encoding means tomodulation symbols, and an optical modulation means for modulating theoptical carrier wave based on symbol signals output from the mappingmeans and outputting an optical modulated signal, wherein the opticalreceiver includes a photoelectric conversion means for receiving andconverting the optical modulated signal into an electrical signal andoutputting a received signal, a demapping means for demapping thereceived signal and outputting a received bit signal, a decoding meansfor receiving input of the received bit signal and decoding the receivedbit signal by using one of a plurality of decoding methods, and adecoding control means for selecting a predetermined decoding methodfrom among the plurality of decoding methods and causing the decodingmeans to operate in accordance with the predetermined decoding method.

An optical communication method according to an exemplary aspect of thepresent invention includes encoding digital signals to be transmittedunder a predetermined transmission condition over an optical carrierwave by selecting a predetermined encoding method corresponding to thepredetermined transmission condition; generating symbol signals bymapping encoded bit signals to modulation symbols; and generating anoptical modulated signal obtained by modulating the optical carrier wavebased on the symbol signals.

Advantageous Effects of Invention

According to the optical transmitter, the optical communication system,and the optical communication method of the present invention, it ispossible to achieve low power consumption and easy control even though aconfiguration is employed in which modulation schemes used for anoptical communication system can be switched depending on transmissionconditions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an opticaltransmitter according to a first example embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating a configuration of an opticaltransmitter according to a second example embodiment of the presentinvention.

FIG. 3A is a constellation diagram to explain a case whereset-partitioning is used as an encoding method for a 16QAM signal in theoptical transmitter according to the second example embodiment of thepresent invention, and a constellation diagram in accordance withordinary 16QAM modulation.

FIG. 3B is a constellation diagram to explain a case whereset-partitioning is used as an encoding method for a 16QAM signal in theoptical transmitter according to the second example embodiment of thepresent invention, and a constellation diagram in accordance withSP8-16QAM modulation.

FIG. 4 is a block diagram illustrating a configuration of an opticaltransmitter according to a third example embodiment of the presentinvention.

FIG. 5 is a block diagram illustrating a configuration of aconvolutional encoder included in the optical transmitter according tothe third example embodiment of the present invention.

FIG. 6 illustrates constellation diagrams to explain the operation ofthe optical transmitter according to the third example embodiment of thepresent invention.

FIG. 7 is a block diagram illustrating a configuration of an opticalcommunication system according to a fourth example embodiment of thepresent invention.

FIG. 8 is a block diagram illustrating a configuration of an opticaltransmitter according to a fifth example embodiment of the presentinvention.

FIG. 9 is a block diagram illustrating a configuration of an encoderincluded in the optical transmitter according to the fifth exampleembodiment of the present invention.

FIG. 10 illustrates logical formulae to explain the operation of asecond encoder included in the optical transmitter according to thefifth example embodiment of the present invention.

FIG. 11 is a constellation diagram of 16QAM to explain the operation ofthe optical transmitter according to the fifth example embodiment of thepresent invention.

FIG. 12 is a constellation diagram of 12QAM to explain the operation ofthe optical transmitter according to the fifth example embodiment of thepresent invention.

FIG. 13A illustrates a set of constellation diagrams of 12QAM in Xpolarization and Y polarization to explain the operation of the opticaltransmitter according to the fifth example embodiment of the presentinvention.

FIG. 13B illustrates another set of constellation diagrams of 12QAM in Xpolarization and Y polarization to explain the operation of the opticaltransmitter according to the fifth example embodiment of the presentinvention.

FIG. 13C illustrates yet another set of constellation diagrams of 12QAMin X polarization and Y polarization to explain the operation of theoptical transmitter according to the fifth example embodiment of thepresent invention.

FIG. 13D illustrates yet another set of constellation diagrams of 12QAMin X polarization and Y polarization to explain the operation of theoptical transmitter according to the fifth example embodiment of thepresent invention.

EXAMPLE EMBODIMENT

Example embodiments of the present invention will be described belowwith reference to the drawings. The directions of arrows in the drawingsare illustrative and do not limit the directions of signals betweenblocks.

A First Example Embodiment

FIG. 1 is a block diagram illustrating a configuration of an opticaltransmitter 100 according to a first example embodiment of the presentinvention. The optical transmitter 100 includes an encoder 110, anencoding control unit 120, a mapping unit 130, and an optical modulationunit 140.

The encoder 110 encodes digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave byusing one of a plurality of encoding methods. The encoding control unit120 selects a predetermined encoding method corresponding to thepredetermined transmission condition from among the plurality ofencoding methods and causes the encoder 110 to operate in accordancewith the predetermined encoding method. The mapping unit 130 maps outputbit signals output from the encoder 110 to modulation symbols. Theoptical modulation unit 140 modulates the optical carrier wave based onsymbol signals output from the mapping unit 130.

Next, the operation of the optical transmitter 100 according to thepresent example embodiment will be described.

M-bit digital signals a₁-a_(m) inputted into the optical transmitter 100are inputted into the encoder 110. The encoder 110 encodes the digitalsignals in accordance with an encoding method set by the encodingcontrol unit 120, and outputs n-bit bit sequence b₁-b_(n) of output bitsignals. The mapping unit 130 maps the bit sequence b₁-b_(n) to symbolsand outputs a data sequence of p elements (dimensions), S₁, S₂, . . . ,S_(p), each of which is a symbol signal, to the optical modulation unit140.

The optical modulation unit 140 performs optical modulation based onrespective data of S₁, S₂, . . . , S_(p) and outputs a transmissionoptical signal on which an optical coded modulation has been performed.The optical modulation unit 140 includes a D/A converter(digital-to-analog converter), a modulator driver, an optical modulator,and a light source, which are not shown in the figure.

The encoder 110 is capable of selecting and setting a predeterminedencoding method from among k encoding methods, from encoding method 1 toencoding method k, as illustrated in FIG. 1. The encoder 110 may beconfigured to perform encoding by using one of a plurality ofconvolutional encoding methods that differ in at least one of agenerating polynomial, a constraint length, and a code rate inconvolutional encoding methods. In this case, the encoding control unit120 selects the predetermined encoding method by setting at least one ofthe generating polynomial, the constraint length, and the code rate. Inother words, the encoding control unit 120 can select and set apreferable encoding method from among encoding method 1 to encodingmethod k depending on a predetermined transmission condition such as atransmission distance and a transmission capacity that is required foreach communication.

At least one of a transmission capacity, a transmission distance, anerror rate, and an optical signal-to-noise ratio can be used as theabove-mentioned transmission condition.

The symbol signals with which the optical modulation unit 140 is driven,that is, a data sequence of S₁, S₂, . . . , S_(p), can be signals withat least one dimension of dimensions including optical phase (Icomponent and Q component), polarization (X polarization component and Ypolarization component), wavelength of the optical carrier wave, andtime. It is possible to perform high-dimensional optical codedmodulation by combining the plurality of dimensions.

An optical modulator constituting the optical modulation unit 140 iscomposed of a material including at least one of a ferroelectricmaterial such as lithium niobate (LiNbO₃), and a semiconductor material.By using the optical modulator alone or those optical modulators incombination, digital signals can be transmitted multiplexed by at leastone of polarization multiplexing, wavelength multiplexing, andtime-division multiplexing.

Next, an optical communication method according to the present exampleembodiment will be described.

In the optical communication method according to the present exampleembodiment, first, digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave areencoded by selecting a predetermined encoding method corresponding tothe predetermined transmission condition. Then symbol signals aregenerated by mapping encoded bit signals to modulation symbols. Lastly,an optical modulated signal is generated that is obtained by modulatingthe optical carrier wave based on the symbol signals.

As mentioned above, the optical transmitter 100 and the opticalcommunication method according to the present example embodiment areconfigured to encode digital signals by selecting a predeterminedencoding method corresponding to a predetermined transmission condition.This configuration makes it possible to select a preferable modulationscheme corresponding to a predetermined transmission condition only bychanging an encoding method. This enables a change in digital signalprocessing to be minimized. As a result, it is possible to achieve lowpower consumption and easy control even though a configuration isemployed in which modulation schemes used for an optical communicationsystem can be switched depending on transmission conditions.

A Second Example Embodiment

Next, a second example embodiment of the present invention will bedescribed. FIG. 2 is a block diagram illustrating a configuration of anoptical transmitter 200 according to the second example embodiment ofthe present invention.

The optical transmitter 200 includes an encoder 110, an encoding controlunit 120, a mapping unit 230, and an optical modulation unit 140. Themapping unit 230 in the optical transmitter 200 according to the presentexample embodiment includes a set-partitioning unit 231 and a symbolselection unit 232. All remaining configurations are the same as thoseof the optical transmitter 100 according to the first exampleembodiment, so their detailed description will be omitted.

The set-partitioning unit 231 partitions modulation symbols into aplurality of subsets and selects a subset included in the plurality ofsubsets based on output bit signals. The symbol selection unit 232selects one modulation symbol from among the modulation symbols includedin a selected subset that is selected by the set-partitioning unit 231based on the output bit signals and maps the output bit signals to aselected modulation symbol.

Next, the operation of the optical transmitter 200 according to thepresent example embodiment will be described.

The encoder 110 receives input of m-bit digital signals a₁-a_(m)inputted into the optical transmitter 200. The encoder 110 encodes thedigital signals in accordance with an encoding method that is set by theencoding control unit 120, and then outputs an n-bit bit sequenceb₁-b_(n) of output bit signals.

The bit sequence b₁-b_(n) is mapped to symbols in the mapping unit 230,and then inputted into the optical modulation unit 140 as data sequencesof XI-ch and XQ-ch that are optical phase components of X polarizationand data sequences of YI-ch and YQ-ch that are optical phase componentsof Y polarization, for example. The optical modulation unit 140 performsoptical modulation based on respective data of XI-ch, XQ-ch, YI-ch, andYQ-ch, and outputs a transmission optical signal on which an opticalcoded modulation has been performed.

The set-partitioning unit 231 partitions a constellation of atwo-dimensional or higher dimensional QAM modulation into Lsub-constellations (states) based on the set-partitioning method. Thenthe set-partitioning unit 231 selects one of the above-mentioned Lsub-constellations (states) using an encoded α bits in the bit sequenceb₁-b_(n). The symbol selection unit 232 selects one preferable symbolfrom among the selected sub-constellation using β bits that have notbeen encoded and are included in the bit sequence b₁-b_(n), and outputsdata corresponding to XI-ch, XQ-ch, YI-ch, and YQ-ch.

The set-partitioning (SP) method described above is the method that theminimum distance between codes is extended by thinning out nearestneighboring points from symbol points. For example, Non PatentLiterature 1 discloses an SP-16QAM modulation in which symbolpartitioning in accordance with the set-partitioning is performed onsymbols that are mapped into a four-dimensional signal space of phaseinformation and polarization information. The distance between symbolsis extended by using the set-partitioning, which enables receivingsensitivity to improve.

Next, the operation of the encoder 110 will be specifically described.

FIG. 3A and FIG. 3B are constellation diagrams to explain a case wherethe above-mentioned set-partitioning is used as the encoding method fortwo-dimensional 16QAM signals. FIG. 3A is a constellation diagram ofordinary 16QAM, where the minimum distance between symbols isrepresented by d_(min). FIG. 3B is a constellation diagram illustratingsymbol mapping by using SP8-16QAM encoding modulation where symbols arealternately thinned out from the 16QAM constellation illustrated in FIG.3A. It can be seen that, in this case, the receiving sensitivityimproves because the minimum distance between symbols expands to 2^(1/2)d_(min).

In the SP8-16QAM encoding method illustrated in FIG. 3B, the code rate“r” reduces by half to ½ because symbols are thinned out. Accordingly,the transmission rate decreases. Consequently, the encoding control unit120 can select the encoding method by ordinary 16QAM illustrated in FIG.3A for the application to which the transmission rate (capacity) is moreimportant. On the other hand, to the application requiring long-distancetransmission, the encoding control unit 120 can select the encodingmethod by SP8-16QAM illustrated in FIG. 3B. In this way, the encodingcontrol unit 120 can select an encoding method corresponding to atransmission condition from among encoding methods such as ordinary16QAM (SP16-16QAM) without thinning out a symbol, SP8-16QAM describedabove, and SP4-16QAM obtained by further thinning out symbols fromSP8-16QAM.

In the above description, the set-partitioning in two-dimensional 16QAMon a simple I-Q plane has been described for simplicity. Without beinglimited to this, it is possible to use other modulation schemes such asfour-dimensional PM-16QAM expanded by adding two dimensions of Xpolarization and Y polarization using polarization multiplexing (PM).

In the case of four-dimensional PM-16QAM, because symbol points can becreated by combining 16 symbols for X polarization (XI, XQ) and 16symbols for Y polarization (YI, YQ), there are 256 (=16×16) symbolpoints. Accordingly, ordinary PM-16QAM without thinning out a symbol canbe described as SP256-PM-16QAM, SP128-PM-16QAM can be obtained bythinning outing symbol points alternately from SP256-PM-16QAM, andSP64-PM-16QAM, SP32-PM-16QAM and so on can be obtained by furtherthinning out symbol points.

In this case, the encoding control unit 120 can select an encodingmethod such as SP32-PM-16QAM and SP128-PM-16QAM, for example. That is tosay, it is possible to make encoding methods (encoding method 1 toencoding method k) in the encoder 110 correspond to PM-16QAM,SP32-PM-16QAM, SP128-PM-16QAM, and so on.

The set-partitioning unit 231 selects one subset by selecting modulationsymbols that correspond to one of polarization states of the opticalcarrier wave based on the output bit signals (a bit sequence b₁-b_(n)).

Because encoding method 1 to encoding method k described above can beset with their receiving sensitivities and code rates differing, itbecomes possible to select a modulation scheme using a preferableencoding method depending on the transmission distance and thetransmission capacity to be required.

Based on the constellation by ordinary 16QAM modulation, a preferablemodulation scheme can be selected simply by changing an encoding methodof the encoder 110 from encoding method 1 to encoding method k. Thisenables a change in digital signal processing to be minimized. As aresult, it is possible to achieve low power consumption and easy controleven though a configuration is employed in which modulation schemes usedfor an optical communication system can be switched depending ontransmission conditions. In addition, because physical interfaces suchas an optical modulator can be shared among respective encoding methods,the component count can be decreased. This also makes it possible toachieve cost reduction and easy control.

Next, an optical communication method according to the present exampleembodiment will be described.

In the optical communication method according to the present exampleembodiment, first, digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave areencoded by selecting a predetermined encoding method corresponding tothe predetermined transmission condition. Then symbol signals aregenerated by mapping encoded bit signals to modulation symbols. Lastly,an optical modulated signal is generated that is obtained by modulatingthe optical carrier wave based on the symbol signals.

In generating the symbol signals mentioned above, the modulation symbolsare partitioned into a plurality of subsets, and a subset included inthe plurality of subsets is selected based on the bit signals. Onemodulation symbol is selected from among modulation symbols included ina selected subset that has been selected based on the bit signals, andthe bit signals can be mapped to a selected modulation symbol.

According to the optical communication method of the present exampleembodiment, it is possible to achieve low power consumption and easycontrol even though a configuration is employed in which modulationschemes used for an optical communication system can be switcheddepending on transmission conditions.

A Third Example Embodiment

Next, a third example embodiment of the present invention will bedescribed. FIG. 4 is a block diagram illustrating a configuration of anoptical transmitter 300 according to the third example embodiment of thepresent invention.

In the optical transmitter 300 according to the present exampleembodiment, an encoder is a convolutional encoder 310. The convolutionalencoder 310 performs encoding by using one of a plurality ofconvolutional encoding methods that differ in at least one of agenerating polynomial, a constraint length, and a code rate inconvolutional encoding methods. The same configurations as those of theoptical transmitter 200 according to the second example embodimentillustrated in FIG. 2 are represented by the same reference numerals,and the description of the configurations will be omitted.

The operation of the optical transmitter 300 according to the presentexample embodiment will be described below.

The convolutional encoder 310 receives input of m-bit digital signalsa₁-a_(m) inputted into the optical transmitter 300. The convolutionalencoder 310 encodes the digital signals in accordance with an encodingmethod that is set by an encoding control unit 120, and then outputs ann-bit bit sequence b₁-b_(n) of output bit signals.

A mapping unit 230 maps the bit sequence b₁-b_(n) to symbols and outputsa data sequence of p elements (dimensions), S₁, S₂, . . . , S_(p), eachof which is a symbol signal, to an optical modulation unit 140. Theoptical modulation unit 140 performs optical modulation based onrespective data of S₁, S₂, . . . , S_(p) and outputs a transmissionoptical signal on which an optical coded modulation has been performed.

The convolutional encoder 310 is configured to be capable of selectingand setting one of k encoding methods from encoding method 1 to encodingmethod k that differ in generating polynomial, constraint length, coderate or the like. The encoding control unit 120 selects a preferableencoding method corresponding to a transmission distance or transmissioncapacity to be required from among the encoding methods from encodingmethod 1 to encoding method k. The encoding control unit 120 sets theconvolutional encoder 310 to operate in accordance with the selectedencoding method.

FIG. 5 illustrates a configuration of a convolutional encoder with theconstraint length equal to 4 and the code rate equal to ⅔, as an exampleof the convolutional encoder 310. The convolutional encoder 310illustrated in FIG. 5 encodes input a₁ and input a₂ and outputs encodedbits b₁-b₃. The convolutional encoder 310 outputs input a₃-a₇ withoutencoding as b₄-b₈.

FIG. 6 illustrates an example of the set-partitioning where theconvolutional encoder 310 illustrated in FIG. 5 performs theconvolutional encoding on four-dimensional PM-16QAM using optical phase(I, Q) and polarization (X, Y). Although FIG. 6 illustratesconstellations for X polarization only for simplicity, the actualconstellations are four-dimensional constellations that are obtained bycombining the constellations for Y polarization. As illustrated in FIG.6, there are eight states (sub-constellations) composed of S1 to S8resulting from two steps of set-partitioning. Because the constellationis made four dimensional (I, Q, X, Y) by combining a constellation for Ypolarization with each state of S1 to S8, each state includes 2⁵=32symbol points.

Next, the operation of a symbol selection unit 232 using theconvolutional encoder 310 will be described.

The symbol selection unit 232 first selects one state from among theeight states (S1 to S8) using three bits b₁-b₃ that areconvolutional-encoded by the convolutional encoder 310 illustrated inFIG. 5. In addition, the symbol selection unit 232 selects one of 32symbols that are included in one of eight selected states using fivebits (b₄-b₈) without encoding.

In this way, the convolutional-encoding enables the least squaredistance between code sequences to be extended and reach to a distancegreater than or equal to the square distance between signals in thestates partitioned by the set-partitioning. This makes it possible toselect a modulation scheme corresponding to a transmission distance andtransmission capacity to be required simply by changing a setting of theconvolutional encoder 310 having a plurality of convolutional encodingmethods that differ in the constraint length and the code rate, based onPM-16QAM modulation. The convolutional encoding method may be determinedby setting the generating polynomial instead of the constraint lengthand the code rate.

As described above, according to the optical transmitter 300 of thepresent example embodiment, it is possible to achieve low powerconsumption and easy control even though a configuration is employed inwhich modulation schemes used for an optical communication system can beswitched depending on transmission conditions.

A Fourth Example Embodiment

Next, a fourth example embodiment of the present invention will bedescribed. FIG. 7 is a block diagram illustrating a configuration of anoptical communication system 1000 according to the fourth exampleembodiment of the present invention.

The optical communication system 1000 includes an optical transmitter100 configured to send out a modulated optical signal to a communicationchannel (an optical transmission medium) 600 and an optical receiver 400configured to receive the modulated optical signal propagated throughthe communication channel 600.

The optical transmitter 100 includes an encoder 110, an encoding controlunit 120, a mapping unit 130, and an optical modulation unit 140. Theconfiguration and the operation of the optical transmitter 100 are thesame as those of the optical transmitter according to the first exampleembodiment; accordingly, their detailed description will be omitted.

The optical receiver 400 includes a photoelectric conversion unit 410, ademapping unit 420, a decoder 430, and a decoding control unit 440.

The photoelectric conversion unit 410 receives and converts themodulated optical signal into an electrical signal, and outputs areceived signal. The demapping unit 420 demaps the received signal andoutputs a received bit signal. The decoder 430 receives input of thereceived bit signal and decodes the received bit signal by using one ofa plurality of decoding methods. The decoding control unit 440 selects apredetermined decoding method from among the plurality of decodingmethods and causes the decoder 430 to operate in accordance with thepredetermined decoding method.

Next, the operation of the optical communication system 1000 accordingto the present example embodiment will be described. The operation ofthe optical transmitter 100 to output an optical signal on which anoptical coded modulation has been performed is the same as that in thefirst example embodiment; accordingly, the description will be omitted.

The optical signal output from the optical modulation unit 140 in theoptical transmitter 100 passes through the communication channel 600 andis received by the photoelectric conversion unit 410 in the opticalreceiver 400. The photoelectric conversion unit 410 converts thereceived optical signal into an electrical signal and outputs thereceived signal as digital signals in each lane of XI-ch, XQ-ch, YI-ch,and YQ-ch. The photoelectric conversion unit is configured to include a90°-hybrid, a photodiode, a transimpedance amplifier, and an A/D(analog-to-digital) converter, which are not shown in the figure.

The demapping unit 420 performs symbol identification on the datasequences of XI-ch, XQ-ch, YI-ch, and YQ-ch and outputs an n-bit bitsequence c₁-c_(n) as a received bit signal. The bit sequence c₁-c_(n) isinputted into the decoder 430 having a plurality of decoding methods.

The decoder 430 selects one of the plurality of decoding methods inaccordance with a setting by the decoding control unit 440. The decoder430 outputs an m-bit bit sequence d₁-d_(m) of a decoded bit sequence.The Viterbi decoding method, which is a maximum-likelihood decodingmethod, can be used as the above-mentioned decoding method, and thesequential decoding method can be used for a convolutional code with alonger constraint length.

The optical communication system 1000 can further include an opticalnetwork control unit 450. The optical network control unit 450determines the predetermined encoding method and the predetermineddecoding method that correspond to the predetermined transmissioncondition, and concurrently informs the encoding control unit 120 andthe decoding control unit 440 of the predetermined encoding method andthe predetermined decoding method.

Next, the operation of the optical network control unit 450 will bedescribed in further detail.

The optical network control unit 450 selects a preferable encodingmethod and a decoding method based on a transmission distance andcommunication quality information such as a transmission capacity thatare the transmission conditions required by a system operating side.Then the optical network control unit 450 concurrently informs theencoding control unit 120 and the decoding control unit 440 of theselection results. Specifically, the optical network control unit 450instructs the encoding control unit 120 on the setting of a generatingpolynomial, a constraint length, a code rate and the like, and instructsthe decoding control unit 440 on the setting of a constraint length, acode length, a soft-decision bit number and the like. The settings ofthe encoding control unit 120 and the decoding control unit 440 areconcurrently changed, which makes it possible to maintain a preferablereception condition.

It is not necessarily required for the optical network control unit 450to obtain the above-mentioned communication quality information used forthe control from the system operating side. It is possible to select apreferable coding method and decoding method using information such asan optical signal-to-noise ratio and an error rate, for example.

Next, an optical communication method according to the present exampleembodiment will be described.

In the optical communication method according to the present exampleembodiment, first, digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave areencoded by selecting a predetermined encoding method corresponding tothe predetermined transmission condition. Then symbol signals aregenerated by mapping encoded bit signals to modulation symbols. Anoptical modulated signal is generated that is obtained by modulating theoptical carrier wave based on the symbol signals.

Next, the modulated optical signal is received, and a received signal isgenerated by converting the modulated optical signal into an electricalsignal. A received bit signal is generated by demapping the receivedsignal. Lastly, the received bit signal is decoded by using apredetermined decoding method selected from among the plurality ofdecoding methods.

As mentioned above, according to the optical communication system 1000and the optical communication method of the present example embodiment,it is possible to achieve low power consumption and easy control eventhough a configuration is employed in which modulation schemes used foran optical communication system can be switched depending ontransmission conditions.

A Fifth Example Embodiment

Next, a fifth example embodiment of the present invention will bedescribed. FIG. 8 is a block diagram illustrating a configuration of anoptical transmitter 500 according to the fifth example embodiment of thepresent invention.

The optical transmitter 500 includes an encoder 510, an encoding controlunit 120, a mapping unit 530, and an optical modulation unit 140. Theoptical transmitter 500 according to the present example embodimentdiffers in the configurations and the operations of the encoder 510 andthe mapping unit 530 from the optical transmitter 100 according to thefirst example embodiment that includes the encoder 110 and the mappingunit 130. The other configurations and the operations are the same asthose of the optical transmitter 100 according to the first exampleembodiment; accordingly, their detailed description will be omitted.

A case will be described below where the encoder 510 includes a firstencoder 511, a second encoder 512, and a third encoder 513 thatcorrespond to encoding method 1, encoding method 2, and encoding method3, respectively. In this case, the encoding control unit 120 selects anoptimum encoding method from among encoding method 1, encoding method 2,and encoding method 3, depending on a predetermined transmissioncondition such as a transmission distance and transmission capacity thatis required for communication, and sets operation modes of the encoder510, the mapping unit 530, and the optical modulation unit 140.

Next, the operations of the encoder 510 and the mapping unit 530 will bedescribed in detail.

FIG. 9 illustrates a configuration of the encoder 510. As mentionedabove, the encoder 510 includes three encoders, that is, the firstencoder 511, the second encoder 512, and the third encoder 513. Theseencoders are configured with the number of input bits equal to fivebits, six bits, and seven bits, respectively. The configurations and theoperations of the first encoder 511 and the third encoder 513 aredescribed in Non Patent Literature; accordingly, the detaileddescription of them will be omitted.

The second encoder 512 calculates the exclusive OR of the input with 6bits of b₁-b₆ and outputs the calculated result as b₇. Next, afour-dimensional encoder 512E included in the second encoder 512converts the input with 7 bits of b₁-b₇ into the output with 8 bits.FIG. 10 illustrates specific logical formulae in the four-dimensionalencoder 512E.

The mapping unit 530 receives the outputs with 8 bits from the encoder510 and allocates them to symbols so as to obtain a coding gain. It isassumed that the symbols selected here belong to a four-dimensionalsymbol space. In the present example embodiment, optical phases (Icomponent and Q component) and polarization components (X polarizationcomponent and Y polarization component) of the optical carrier wave areused as the four-dimensional signal space, and symbol signals resultingfrom symbol mapping are output to the optical modulation unit 140.

Specifically, the symbols are allocated using mapping symbolsillustrated in FIG. 11. The mapping unit 530 allocates transmissionsymbols in the X polarization using bits of B₁ to B₄ output from theencoder 510 and allocates transmission symbols in the Y polarizationusing bits of B₅ to B₈. In this case, the mapping unit 530 allocatessymbols so as to produce 12QAM constellation in which the points at thefour corners of 16QAM constellation are eliminated as illustrated inFIG. 12. As can be seen from FIG. 12, the signal points in 12QAMconstellation can be classified into eight points with a large amplitudeand 4 points with a small amplitude.

In addition, the encoder 510 in the optical transmitter 500 according tothe present example embodiment is configured to perform encoding so thatthe amplitude of the optical signal modulated by the optical modulationunit 140 may have a correlation between two types of the polarization (Xpolarization and Y polarization) of the optical carrier wave. Themapping unit 530 is configured to allocate a symbol to a signal pointwhere the amplitude is maximized in at least one polarization state.

The operation of the mapping unit 530 will be described in furtherdetail with reference to constellation diagrams of 12QAM in Xpolarization and Y polarization that are illustrated in FIG. 13A to FIG.13D. As illustrated in FIG. 13A to FIG. 13C, there is a followingcorrelation on amplitudes between symbols in the X polarization andsymbols in the Y polarization. That is to say, in the case illustratedin FIG. 13A, there are symbol points with large amplitude in both the Xpolarization and the Y polarization. In the case illustrated in FIG.13B, there are symbol points with small amplitude in the X polarization,and there are symbol points with large amplitude in Y polarization.Contrary to the above case, in the case illustrated in FIG. 13C, thereare symbol points with large amplitude in the X polarization, and thereare symbol points with small amplitude in the Y polarization. However, aconstellation is excluded in which only symbol points with smallamplitude are included in both the X polarization and the Y polarizationas illustrated in FIG. 13D. In this way, the mapping unit 530 isconfigured to allocate a symbol to a signal point where the amplitude ismaximized in at least one polarization state, that is, the symbolconstellations illustrated in FIG. 13D are excluded, which makes itpossible to decrease the number of adjacent points. This enables the biterror rate to be reduced.

The numbers of signal points illustrated in FIG. 13A, FIG. 13B, and FIG.13C are 64 (=8×8), 32 (=4×8), and 32 (=8×4), respectively; there are 128points in total that constitute four-dimensional symbol points. However,because the second encoder 512 generates parity bits, four-dimensionalset-partitioning is performed, and symbol points are thinned out tohalf; as a result, the number of symbol points generated by the secondencoder 512 and the mapping unit 530 becomes 64. This number isequivalent to that of conventional PM-8QAM.

Here, with Es representing the transmission power of transmission symbolper polarization state, the distance between signal points in 16QAM isequal to 0.63 Es^(1/2) because the Es is the average of squares of theamplitudes of all symbols. In the case of 12QAM, on the other hand, thedistance between signal points increases to 0.71Es^(1/2). This effectresults from eliminating the symbols at the four corners of 16QAM symbolpoints.

In addition, because the second encoder 512 performs thefour-dimensional set-partitioning, the distance between four-dimensionalsignal points in 12QAM constellation increases to 1.0 Es^(1/2) by afactor of 2^(1/2), which is greater than the distance between signalpoints in the conventional PM-8QAM that is equal to 0.92 Es^(1/2).Consequently, the forming of 12QAM constellation can achieve theperformance higher than or equal to that of PM-8QAM.

As mentioned above, using the second encoder 512 according to thepresent example embodiment, the amplitudes are made to correlate witheach other between two types of the polarization of the optical carrierwave, which enables the receiving sensitivity to be improved. It goeswithout saying that the configuration to make the amplitudes betweenpolarizations correlate with each other is applicable to not only theother QAM signals but also every symbol constellation. Thisconfiguration is also applicable to a configuration in which a case isexcluded where there are symbol points with large amplitude in both theX polarization and Y polarization as illustrated in FIG. 13A, instead ofthe above-mentioned configuration in which a case illustrated in FIG.13D is excluded.

Next, an optical communication method according to the present exampleembodiment will be described.

In the optical communication method according to the present exampleembodiment, first, digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave areencoded by selecting a predetermined encoding method corresponding tothe predetermined transmission condition. Then symbol signals aregenerated by mapping encoded bit signals to modulation symbols. Lastly,an optical modulated signal is generated that is obtained by modulatingthe optical carrier wave based on the symbol signals.

As mentioned above, the optical transmitter 500 and the opticalcommunication method according to the present example embodiment areconfigured to encode digital signals by selecting a predeterminedencoding method corresponding to a predetermined transmission condition.This configuration makes it possible to select a preferable modulationscheme corresponding to a predetermined transmission condition only bychanging an encoding method.

It is possible to minimize a change in digital signal processing becausea basic symbol constellation remains unchanged even though encodingmethods are switched. As a result, it is possible to achieve low powerconsumption and easy control even though a configuration is employed inwhich modulation schemes used for an optical communication system can beswitched depending on transmission conditions. It is also possible toachieve an effect of reducing the bit error rate and extending thetransmission distance. In addition, because physical interfaces such asan optical modulator can be shared among respective encoding methods,the component count can be decreased. This also makes it possible toachieve cost reduction and easy control.

The present invention has been described by taking the exampleembodiments mentioned above as model examples. However, the presentinvention is not limited to the above-mentioned example embodiments.Various modes that can be understood by those skilled in the art can beused within the scope of the present invention.

This application is based upon and claims the benefit of priority fromthe Japanese Patent Application No. 2014-209346, filed on Oct. 10, 2014,and Japanese Patent Application No. 2015-088334, filed on Apr. 23, 2015,the disclosure of which is incorporated herein in its entirety byreference.

REFERENCE SIGNS LIST

-   100, 200, 300, 500 Optical transmitter-   110, 510 Encoder-   120 Encoding control unit-   130, 230, 530 Mapping unit-   140 Optical modulation unit-   231 Set-partitioning unit-   232 Symbol selection unit-   310 Convolutional encoder-   400 Optical receiver-   410 Photoelectric conversion unit-   420 Demapping unit-   430 Decoder-   440 Decoding control unit-   450 Optical network control unit-   511 First encoder-   512 Second encoder-   512E Four-dimensional encoder-   513 Third encoder-   600 Communication channel-   1000 Optical communication system

1. An optical transmitter, comprising: an encoder configured to encodedigital signals to be transmitted under a predetermined transmissioncondition over an optical carrier wave by using one of a plurality ofencoding methods; an encoding control unit configured to select apredetermined encoding method corresponding to the predeterminedtransmission condition from among the plurality of encoding methods andcausing the encoder to operate in accordance with the predeterminedencoding method; a mapping unit configured to map output bit signalsoutput from the encoder to modulation symbols; and an optical modulationunit configured to modulate the optical carrier wave based on symbolsignals output from the mapping unit.
 2. The optical transmitteraccording to claim 1, wherein the mapping unit includes aset-partitioning unit and a symbol selection unit, the set-partitioningunit partitions the modulation symbols into a plurality of subsets andselects a subset included in the plurality of subsets based on theoutput bit signals, and the symbol selection unit selects one modulationsymbol from among modulation symbols included in a selected subsetselected by the set-partitioning unit based on the output bit signalsand maps the output bit signals to a selected modulation symbol.
 3. Theoptical transmitter according to claim 1, wherein the encoder performsencoding by using one of a plurality of convolutional encoding methodsthat differ in at least one of a generating polynomial, a constraintlength, and a code rate in convolutional encoding methods; and theencoding control unit selects the predetermined encoding method bysetting at least one of the generating polynomial, the constraintlength, and the code rate.
 4. The optical transmitter according to claim2, wherein the set-partitioning unit selects the subset by selecting amodulation symbol that corresponds to one of polarization states of theoptical carrier wave based on the output bit signals.
 5. The opticaltransmitter according to claim 1, wherein the transmission condition isa condition on at least one of a transmission capacity, a transmissiondistance, an error rate, and an optical signal-to-noise ratio.
 6. Theoptical transmitter according to claim 1, wherein the symbol signals arecomposed of signals with at least one dimension of dimensions includingoptical phase, polarization, and wavelength of the optical carrier wave,and time.
 7. The optical transmitter according to claim 6, wherein theencoder performs encoding in such a way that an amplitude of an opticalsignal modulated by the optical modulation unit has a correlationbetween two types of polarization states of the optical carrier wave. 8.The optical transmitter according to claim 1, wherein an opticalmodulator constituting the optical modulation unit is composed of amaterial including at least one of a ferroelectric material and asemiconductor material.
 9. The optical transmitter according to claim 1,wherein the digital signals are multiplexed by using at least one ofpolarization multiplexing, wavelength multiplexing, and time-divisionmultiplexing, and transmitted.
 10. An optical communication system,comprising: an optical transmitter configured to send out a modulatedoptical signal to an optical transmission medium; and an opticalreceiver configured to receive the modulated optical signal propagatedthrough the optical transmission medium, wherein the optical transmitterincludes an encoder configured to encode digital signals to betransmitted under a predetermined transmission condition over an opticalcarrier wave by using one of a plurality of encoding methods, anencoding control unit configured to select a predetermined encodingmethod corresponding to the predetermined transmission condition fromamong the plurality of encoding methods and causing the encoder tooperate in accordance with the predetermined encoding method, a mappingunit configured to map output bit signals output from the encoder tomodulation symbols, and an optical modulation unit configured tomodulate the optical carrier wave based on symbol signals output fromthe mapping unit and outputting an optical modulated signal, wherein theoptical receiver includes a photoelectric conversion unit configured toreceive and convert the optical modulated signal into an electricalsignal and outputting a received signal, a demapping unit configured todemap the received signal and output a received bit signal, a decoderconfigured to receive input of the received bit signal and decode thereceived bit signal by using one of a plurality of decoding methods, anda decoding control unit configured to select a predetermined decodingmethod from among the plurality of decoding methods and cause thedecoder to operate in accordance with the predetermined decoding method.11. The optical communication system according to claim 10, furthercomprising an optical network control unit, wherein the optical networkcontrol unit determines the predetermined encoding method and thepredetermined decoding method that correspond to the predeterminedtransmission condition and concurrently informs the encoding controlunit and the decoding control unit of the predetermined encoding methodand the predetermined decoding method.
 12. The optical communicationsystem according to claim 11, wherein the optical network control unitinforms the encoding control unit of, based on the determination, atleast one of a generating polynomial, a constraint length, and a coderate in convolutional encoding methods, and informs the decoding controlunit of, based on the determination, the constraint length and asoft-decision bit number.
 13. The optical communication system accordingto claim 10, wherein the decoding method is one of a Viterbi decodingmethod and a sequential decoding method.
 14. The optical communicationsystem according to claim 10, wherein the transmission condition is acondition on at least one of a transmission capacity, a transmissiondistance, an error rate, and an optical signal-to-noise ratio.
 15. Theoptical communication system according to claim 10, wherein the symbolsignals are composed of signals with at least one dimension ofdimensions including optical phase, polarization, and wavelength of theoptical carrier wave, and time.
 16. An optical communication methodcomprising: encoding digital signals to be transmitted under apredetermined transmission condition over an optical carrier wave byselecting a predetermined encoding method corresponding to thepredetermined transmission condition; generating symbol signals bymapping encoded bit signals to modulation symbols; and generating anoptical modulated signal obtained by modulating the optical carrier wavebased on the symbol signals.
 17. The optical communication methodaccording to claim 16, further comprising partitioning the modulationsymbols into a plurality of subsets and selecting a subset included inthe plurality of subsets based on the bit signals, and selecting onemodulation symbol from among modulation symbols included in a selectedsubset having been selected based on the bit signals and mapping the bitsignals to a selected modulation symbol.
 18. The optical communicationmethod according to claim 16, further comprising receiving the opticalmodulated signal and generating a received signal by converting theoptical modulated signal into an electrical signal, generating areceived bit signal by demapping the received signal, and decoding thereceived bit signal by using a predetermined decoding method selectedfrom among the plurality of decoding methods.
 19. The opticalcommunication method according to claim 16, wherein the transmissioncondition is a condition on at least one of a transmission capacity, atransmission distance, an error rate, and an optical signal-to-noiseratio.
 20. The optical communication method according to claim 16,wherein the symbol signals are composed of signals with at least onedimension of dimensions including optical phase, polarization, andwavelength of the optical carrier wave, and time.